http://vision.unipv.it/ PROTEIN STRUCTURE ANALYSIS THROUGH HOUGH - - PowerPoint PPT Presentation

PROTEIN STRUCTURE ANALYSIS THROUGH HOUGH TRANSFORM AND RANGE TREE

VIRGINIO CANTONI, Dipartimento di Informatica e Sistemistica, Università di PAVIA, virginio.cantoni@unipv.it ELIO MATTIA, Center for Systems Chemistry, Rijksuniversiteit Groningen, Groningen, The Netherlands, E.Mattia@rug.nl

http://vision.unipv.it/

Overview

• Searching in a database of protein

structures

– Pairwise comparison – All-to-All comparison – Search for a structural “motif”

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General Hough transform approach to protein structure comparison 3C Vision

cues, contexts and channels Elsevier (April 2011)

• V. Cantoni, S. Levialdi, B. Zavidovique

Università di Pavia, Roma, Université de Paris XI

Image plane

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Paul Hough , 1959 : straight lines

q = yi-mxi q y x y = mx+q m

•  < m, q < +

r = x cos(q) + y sin(q) 0 < r < L2; -  q   Parameter space

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Example

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Richard Duda and Peter Hart 1972: Circles

y x yc xc

dyi/dxi

y

(xc,yc)

• x
• yc= -1/mi xc + (yi- mixi)

f((x,y),xc,yc,r) = (y-yc)2+(x-xc)2-r2=0

Image plane Parameter spaces Parameter space

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Exemple de vote: cercle

Dana H. Ballard 1981: Generalized HT

Mapping rule X0 = x + r cos(a); Y0 = y + r sin(a)

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Exemple de vote : clé 1

Basics on Proteins

• A protein is an ordered sequence of amino

acids

• Building blocks: 20 amino acid residues.
• Three-dimensional shapes (“fold”) vary

enormously.

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Levels of protein structure representation

• Primary structure
• Secondary structure
• Tertiary structure
• Quaternary structure

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Primary structure: the sequence of amino acids

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Secondary structures

Three basic components:

• helix
• sheet
• Loops (linear connections between the

components)

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The helix

• One of the most

closely packed arrangement of residues.

• ~40% of residues

in globular proteins

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The sheet

loosely packed arrangement of residues. Antiparallel Parallel Twisted

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Secondary Structures Representation

• Secondary structures are represented as

linear vectors (segments): the axis for the alpha helix and the best fit segment for a strand

• An alignment algorithm is used to match an

helix segments with known axes to determine helix axis. Direct segment fits are made to fit sheet strands.

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Secondary Structure Determination

• Programs: DSSP and STRIDE.
• On the average 4.8% of the target

residues were differently assigned, this number reaching 12% for certain targets.

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Distribution of segment lenght

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Protein Structure Comparison

What are the most similar folds ? Given a motif or domain or protein

PDB

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Secondary structure representation

• Each segment is

associated to a secondary structure and is displayed as a cylinder

• The protein is

represented by and

• rdered sequence of

cylinder with two labels: helices or strands

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GHT applied to proteins

• For every protein, the distance (r) of

every secondary structure from a reference point (RP, eg the geometric center of the protein) and the angle (theta) between the direction of the secondary structure in the 3D space and the segment linking the center of that secondary structure with the RP are first

• calculated. (GH reference table RT)

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In the way of GHT (simplified 2D representation)

Query protein (scaled 0.5) Mapping Rule Votes Space helices and strands

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In the way of GHT

Query protein Mapping Rule Votes Space helices and strands

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Proteins: the 3D solution

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GH parameters spaces

Credits: Elio Mattia

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GHT applied to proteins

• In the 3D space of a given “object

protein”, every secondary structure of a “model protein” votes a circumference of points starting from every secondary structure of the object protein.

• If the proteins are similar in shape, the

circumferences will all intersect in a given point.

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Main characteristics

• the mapping rule, for each compatible

correspondent, in 3D is a circle on a plane perpendicular to the axis of the secondary structure

• Other information can be exploited to

increase the S/N ratio:

– the length of the secondary structure – the residues properties contained in the SS – any other (biochemical, morphological, etc.) peculiarities.

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The implementation

• The voting space is smoothed by accumulation
• f nearby votes (within a given radius) for

each point

• After smoothing, the highest peaks in the

voting space are detected (avoiding to pick high votes that however are not the top of a peak but lie close to one such peak)

• Only the relevant votes are stored in memory:

there isn’t a matrix with all the possible cells.

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Smoothing Algorithm

• Smoothing is performed by accumulating votes within a

given radius, for every point in the vote space.

• The classic version, i.e., checking every vote for the

vicinity condition, has been proven to be too time- consuming for applications, with a time complexity of O(n2), where n is the number of votes in the vote space.

• The smoothing problem can be seen as an “orthogonal

search” problem, i.e., finding points within a given cube in space.

• A particular structure has been implemented for solving

this problem with a O(n log3(n)) complexity: Range Trees.

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, i c h i I Y - range tree S X - range tree S

Ortogonal range tree

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Ortogonal range tree

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The implementation

• The comparison of ONE (1) object protein

with MANY (N) model proteins is accomplished by sorting the votes of the top peaks in the spaces of each of the (N) model proteins.

• The sorting is carried out in TWO ways:

either the smoothed votes themselves are sorted, or the differences between the two highest peaks in each of the (N) voting spaces are sorted.

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First results

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Testing on Motif Retrieval

• The developed algorithm makes a new approach for

protein structural comparison available.

• The main application of this new approach is to

classify protein structures and to retrieve structural motifs which are common of a given protein function.

• Indeed, tests were performed on motif retrieval.
• As an example, a motif (present in the Ubiquitin

Conjugating Enzyme) was found in other proteins which are known to contain it.

• Further testing will be done with the parallel

implementation of the software.

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Much experimentation allowed

• Computationally, the results might vary

substantially if any of the following parameters are varied:

– The mesh of the voting space (in Ångström) – The mesh of the voting circumference (how many votes in each circumference) – The radius of smoothing – The radius of tolerance for avoiding “false peaks” when detecting peaks – The normalization factor (linear, square root, etc.)

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